Plasminogen activator inhibitor-1 (PAI-1) is a member of the family of endogenous serine protease inhibitors (serpins) that controls the activation of plasminogen into plasmin by tissue and urokinase type plasminogen activators (tPA and uPA, respectively). In the past, PAI-1 was considered to be anti-tumorigenic. However, in the late 1990's multiple clinical studies revealed that cancer patients whose tumors have a high content of uPA and PAI-1 have a poor rather than a favorable clinical outcome, suggesting that PAI-1 may promote rather than inhibit tumorigenesis. One explanation for the paradoxical role attributed to PAI-1 in cancer has been its pro-angiogenic function. During the past funding period of the grant, we have published two fundamental observations that further elucidate the pro-tumorigenic function of PAI-1. We first reported that PAI-1 exerts its pro-angiogenic activity in part through a novel mechanism whereby PAI-1 protects endothelial cells from Fas-mediated apoptosis through inhibition of the shedding by plasmin of a soluble mFasL pro-apoptotic fragment. Secondly, we demonstrated that PAI-1 also protects tumor cells from apoptosis, but that the mechanism is only partially dependent on plasmin and Fas. Emphasizing the role of the tumor microenvironment, we also demonstrated that stromal-derived PAI-1 compensates for a lack of tumor-derived PAI-1, and that PAI-1 contributes to the recruitment of macrophages by tumor cells. The overarching hypothesis of this application is that PAI-1 exerts its pro-tumorigenic activity through a combination of effects on endothelial cells (pro-angiogenic activity), macrophages (promotion of migration) and tumor cells (promotion of survival) that involves specific PAI-1-receptor interactions. We will test this hypothesis in vitro in several human cancer cell lines of different origins and producing variable amounts of PAI-1, and in vivo by combining genetic approaches with a pharmacologic approach. In Aim 1, we will investigate the mechanism by which the interaction between PAI-1 and one of its cell surface receptors, the low density lipoprotein receptor-like protein 1 (LRP1) signals and protects tumor cells from spontaneous and drug-induced apoptosis. In Aim 2, we will determine whether PAI-1 is necessary to allow tumors to promote angiogenesis, recruit macrophages and exit dormancy in immunodeficient PAI-1 null mice implanted with human tumor cells in which the suppression of PAI-1 expression is controlled by doxycycline. We will also investigate the effect of PAI-1 suppression on tumor initiation in PAI-1 null mice crossed with transgenic NB-Tag mice that have a 100% penetrance in neuroblastoma tumor formation. In Aim 3, we will test the effect of pharmacological inhibition of PAI-1 by newly developed small molecule inhibitors of PAI-1 in pre-clinical mouse models of tumorigenesis and tumor initiation. These studies will provide not only a better fundamental understanding of the mechanisms responsible for the pro-tumorigenic activity of PAI-1, but also a more definitive answer on the potential therapeutic value of targetin PAI-1 as part of cancer treatment strategies.